make and example for me ?

In order to establish a causal association (i.e. that the explanatory variable causes the effects we see in the response variable) it is necessary for the study to be a randomized experiment, in which the researchers randomly assign the explanatory variable to the cases. Give a description of a study, and indicate whether it is possible to establish causality based on the methodology of your study, and explain why. You are not required to use a real study; making up a description is fine